I don't think it is sitemap resubmitting lead to SERP loss since I also use an automatic sitemap generating and submitting script in my admin area to re-submit sitemaps when I have new pages added to my sites. Keeping balance between your fresh contents and already ranking pages would be the essential issue.

On my experience I don't have to resubmit sitemap since it will generate automatically. I just tried it. when I checked my SERP's on google 2 days after discovering that SERP's lost, I tried to check again it's SERP's and it came back to normal, so i guess it is because I resubmit the sitemap.xml to GWT?